1. Field of the Invention
The invention relates to a transducer of the accelerometer type having an inertial mass and a piezoelectric bending element.
2. Description of Related Art
Accelerometers are particular in that it is the reaction from an inertial mass due to the movement of the housing which is measured, and there is hence no need for a fixed reference with respect to which a vibration is measured, such as in a microphone. It is at all times possible, by single or double integration, to obtain a function of time which corresponds to the velocity or position in the space where the measurement takes place.
Known accelerometer constructions use discs of piezoelectric material which build up a potential difference between the two sides by compression, by bending or by shear forces. In that the accelerometer comprising the housing and inertial mass contributes a mass which influences the measurement site, a construction becomes a compromise between sensitivity and in particular the inertial mass, because the mass of the housing must be minimised. For instance, an accelerometer for measuring acoustic signals on the human body may in practice frequently have a total mass approaching 30 g (0.03 kg), which can change the measured signal completely. Furthermore, many accelerometers are sensitive to other movements than those which occur along the axis of the accelerometer, thereby causing undesired signals. It is known to compensate certain directions of sensitivity by using a number of piezoelectric elements electrically connected to each other in a suitable fashion, such as DELTA-SHEAR (trademark) which has piezoelectric elements fitted to a prism with an equilateral triangle as a base and with individual inertial masses fitted to the far side. In this case the sensitivity is predominantly axial. However, this type is not suited for very low frequencies and low accelerations, whereas it is very well suited for extremely high accelerations.